Photolithography process and photomask structure implemented in a photolithography process

ABSTRACT

In a photolithography process, a photoresist layer is formed on a substrate. A photomask is aligned over the substrate to transfer pattern images defined in the photomask on the substrate. The photomask includes first and second patterns of different light transmission rates, and a dummy pattern surrounding the second pattern having a light transmission rate lower than that of the first pattern. The substrate is exposed to a light radiation through the photomask. The photoresist layer then is developed to form the pattern images. The dummy pattern is dimensionally configured to allow light transmission, but in a substantially amount so that the dummy pattern is not imaged during exposure.

RELATED APPLICATION

This application claims the priority benefit of Taiwan PatentApplication No. 092135864, filed on Dec. 17, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a semiconductor process, and moreparticularly to a photolithography process and a photomask structurethat can improve the dimensional transfer of pattern images from aphotomask onto a substrate in the photolithography process.

2. Description of the Related Art

In the manufacture of a semiconductor device, a photolithography processis commonly conducted to transfer or image specific patterns predefinedon a photomask to a semiconductor wafer or substrate. For this purpose,a photosensitive material (also called photoresist) is initially formedon the substrate. The photoresist layer can be either a positivephotoresist or negative photoresist layer depending on whether anassigned region is to be removed after development. Then, thephotoresist layer is exposed through the photomask to specific lightradiation, so that the pattern defined on the photomask is imaged on thephotoresist layer. The radiation used during exposure can include deepultraviolet (DUV), X rays, electronic beams, ion beams or the like.After the exposure is completed, the substrate is processed to developthe transferred pattern and thereby form a patterned photoresist layerthrough which the substrate is etched to form the desired circuitpattern.

Since the circuit patterns become increasingly smaller, aphotolithography process of higher precision is required. By reducingthe wavelength of the exposure light in the photolithography process,small active devices and transistors can be realized by establishingsmall critical dimension. The critical dimension of the circuit patternis defined as the smallest pattern line width.

A photomask conventionally includes one or more circuit patterns. Thephotomask can be of positive or negative type depending on whether thepattern images formed on the photomask are opaque or not. The negativephotomask is usually preferred because light scattering in the negativephotomask is in a smaller amount and particles falling in opaque regionsare less likely to be developed. When light strikes fine particles, theparticles absorb light energy. A part of the light energy is absorbedand becomes an internal energy, while the other part of the light energyemerges out under the form of light scattering. Light scattering is oneissue to be solved in optical exposure. During the exposure, theparticles contained in the photomask scatter light so that the criticaldimension of the photoresist pattern formed on the substrate is biasedwith from the dimension set in the photomask pattern.

FIG. 1 is a schematic view of a conventional photomask. A conventionalphotomask 100 can include a first pattern 110 and a second pattern 120of different sizes and an opening area 130. The opening area 130 islocated between the first pattern 110 and the second pattern 120 andsurrounds the second pattern 120. In FIG. 1, the first pattern 110 andthe second pattern area 120 exemplary include line-shaped patterns andhave similar line widths 112, 122 and inter-line pitches 114, 124, butthe first pattern 110 occupies an area larger than the second pattern120. In other words, the first pattern 110 has higher pattern densitythan that of the second pattern 120, i.e. the light transmission rate ishigher through the first pattern 110 than through the second pattern120.

During the exposure, the photomask 100 is placed on a semiconductorsubstrate having a photoresist layer thereon, and a light radiation isprojected through the photomask 100 onto the substrate. The opening area130 may cause light scattering around the second pattern area 120 of thephotomask 100, which may result in the accumulation of light energy inthe underlying photoresist layer.

FIG. 2 is a cross-sectional view of a pattern image obtained by theconventional photolithography process. After development, first andsecond photoresist pattern images 210, 220 are formed on thesemiconductor substrate 10 corresponding to the first and second pattern110, 120 of the photomask 100 as illustrated in FIG. 1. As shown in FIG.2, the photoresist pattern image 220 formed on the substrate 10 hasdimensions biased from those of the corresponding second pattern 120 setin the photomask 100. Reference numeral 222′ indicates the target linewidth as set in the photomask, while reference numeral 222 indicates theactual line width obtained after development. As shown, the line width222 of the photoresist pattern image 220 is smaller than the line width212 of the first photoresist pattern image 210. Since the first andsecond patterns 110, 120 are dimensionally configured with a same linewidth in the photomask 100, the pattern image of the second pattern 120thus has been dimensionally biased in the photolithography process.

Conventionally, the pattern of the photomask having a smaller patterndensity is configured beforehand to compensate the critical dimensionbiases occurring when pattern images of different densities are formed.However, this preliminary compensation becomes difficult to achieve asthe critical dimensions of the semiconductor devices are increasinglyreduced.

Therefore, a need presently exists for a photolithography process thatcan prevent the dimensional biases of pattern images formed in aphotolithography process.

SUMMARY OF THE INVENTION

The application describes a photolithography process using a photomaskthat can prevent critical dimension biases of pattern images ofdifferent pattern densities. A circuit pattern thereby can be formedwithout the need of a conventional compensation process.

In one embodiment, the photolithography process includes the followingsteps. A photoresist layer is formed over a substrate. A photomask isaligned over the substrate, wherein the photomask includes a firstpattern, a second pattern and a dummy pattern around the second pattern.The substrate is exposed to a light radiation through the photomask. Thephotoresist layer then is developed to form images of the first andsecond patterns in the photoresist layer. The dummy pattern isconfigured to allow light transmission in a substantially small amountso that the dummy pattern is not imaged while the first and secondpatterns are transferred on the photoresist layer during exposure.

In one embodiment, the dummy pattern is dimensionally configured toinclude a pattern density substantially smaller than a pattern densityof the first and second patterns. In some embodiments, a line width andan inter-line pitch of the dummy pattern are substantially small so thatthe dummy pattern is not imaged while the first and second patterns aretransferred on the photoresist layer during exposure. In a variantembodiment, the dummy pattern is placed at a distance from the secondpattern to prevent image bias of the second pattern.

The foregoing is a summary and shall not be construed to limit the scopeof the claims. The operations and structures disclosed herein may beimplemented in a number of ways, and such changes and modifications maybe made without departing from this invention and its broader aspects.Other aspects, inventive features, and advantages of the invention, asdefined solely by the claims, are described in the non-limiting detaileddescription set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a conventional photomask;

FIG. 2 is a cross-sectional view of pattern images obtained by aconventional photolithography process;

FIG. 3 is a planar view of a photomask according to one embodiment ofthe invention;

FIG. 4 is a planar view of a photomask according to another embodimentof the invention;

FIG. 5 is a planar view of a photomask according to another variantembodiment of the invention;

FIG. 6 is a cross-sectional view of a substrate provided withphotoresist pattern images obtained by a photolithography process usinga photomask according to an embodiment of the invention; and

FIG. 7 is a flowchart of a photolithography process implementedaccording to an embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

In a photolithography process, a photomask carries patterns of differentsizes and shapes to be transferred on a photoresist layer laid on asubstrate. Due to optical scattering occurring in the exposure step, thecritical size of the pattern images defined in the photoresist layermight experience undesirable biases.

FIG. 3 is a schematic view of a photomask that can alleviate theforegoing problems in a photolithography process according to anembodiment of the invention. The photomask 300 includes at least a firstpattern 310, a second pattern 320, and a dummy pattern 340 around thesecond pattern 320. It is understood that the pattern shown in FIG. 3 isa simplified representation used for purpose of illustration andpractically can be embodied in many forms and shapes. For the purpose ofclarification, “line width” herein refers to either a transversedimension of a photoresist pattern image or the corresponding patterndimension defined in the photomask. “Inter-line pitch” means thedistance between two pattern lines.

In the illustrated embodiment, the first and second patterns 310, 320include linear patterns, and the second pattern 320 occupies an areasmaller than the first pattern 310. The linear first pattern 310includes lines respectively having a line width and an inter-line pitchthat are respectively equal to the line width and inter-line pitch ofthe linear pattern 320. In other words, the first and second patterns310, 320 have different pattern densities that result in differentiallight transmission rates in their respective areas, i.e. the lighttransmission rate is higher through the first pattern 310 than throughthe second pattern 320.

The photomask 300 can be made of adequate materials such as quartz.According to the design demand, the photomask 300 can be made of atransparent or opaque base material. In an embodiment where thephotomask 300 is made of a transparent base material, the first andsecond patterns 310, 320 are opaque. Conversely, if the photomask 300 ismade of an opaque base material, the first, second patterns 310, 320 aretransparent. The first and second patterns 310, 320 can exemplaryinclude circuit patterns implemented to form an integrated circuitdevice.

As shown in FIG. 3, a dummy pattern 340 is formed surrounding the secondpattern 320 of lower light transmission rate. The dummy pattern 340occupies an area larger than the second pattern 320. The dummy pattern340 can be formed in the shape of straight lines, holes, or the like. Inthe illustrated embodiment, the dummy pattern 340 includes a pluralityof opaque straight lines spaced apart from one another and parallel tothe linear second pattern 320.

The dummy pattern 340 is configured to allow light transmission withoutbeing transferred during exposure of the photolithography process. In anembodiment, the inter-line pitch and the line width of the linear dummypattern 340 can be set smaller than those of the second pattern 320. Thepattern density of the dummy pattern 340 has a light transmission ratesufficiently low to prevent its transferring during exposure. The dummypattern 340 is spaced apart from the second pattern at a sufficientdistance to prevent a biased image of the second pattern.

FIG. 4 illustrates a photomask pattern according to a variant embodimentof the invention. The photomask 300 at least includes a first pattern310, a second pattern 320 and a dummy pattern 350 around the secondpattern 320. The first pattern 310 and the second pattern 320 includelines having similar line width and inter-line pitch. The dummy pattern350 surrounds the second pattern 320 and occupies an area larger thanthat of the second pattern 320. The dummy pattern 350 includes aplurality of spaced opaque straight lines, but it is understood thatdiverse shapes of the dummy pattern 350 may be adequate. In theillustrated example, the direction of the linear dummy pattern 350 isapproximately orthogonal to that of the linear second pattern 320. Thedummy pattern 350 is dimensionally configured to allow lighttransmission without being imaged during exposure. To this end, theinter-line pitch and the line width of the linear dummy pattern 340 areset smaller than those of the second pattern 320.

FIG. 5 illustrates another variant embodiment of the invention. Aphotomask 400 is constructed from a base substrate of opaque materialand includes a first pattern 410, a second pattern 420 and a dummypattern 440. The patterns 410, 420, 440 are formed by gap structuressuch as slits or holes allowing light transmission. The first pattern410 occupies an area larger than the second pattern 420, but has thesame line width and the same inter-line pitch as the second pattern 420,i.e. the first and second patterns 410, 420 have different patterndensities that result in a light transmission rate higher through thefirst pattern 410 than through the second pattern 420. The dummy pattern440 surrounds the second pattern 420 and occupies an area larger thanthat of the second pattern 420. The dummy pattern 440 can include spacedlines. In this embodiment, the direction of the linear dummy pattern 440is orthogonal to that of the second pattern 420. The line width and theinter-line pitch of the dummy pattern 440 are smaller than those of thesecond pattern 420 so that the dummy pattern 440 is not transferredduring exposure.

FIG. 7 is a flowchart of a photolithography process implementedaccording to an embodiment of the invention. Initially, a photoresistlayer is formed over a surface of a substrate (702). The photoresistlayer can be either a negative or positive photoresist layer formed byspin-coating. A photomask then is aligned over the substrate (704). Thephotomask includes first and second patterns of different patterndensities to be transferred on the photoresist layer of the substrate,and a dummy pattern surrounding the second pattern of lower patterndensity. The configuration of the dummy pattern can be as described inthe previous embodiments. An exposure then is performed by irradiating alight beam through the photomask (706). The photoresist layer then isdeveloped to form a photoresist pattern (708).

FIG. 6 is a cross-sectional view of photoresist pattern images obtainedby a photolithography process using a photomask according to anembodiment of the invention. The substrate 600 includes a first patternimage 610 and a second pattern image 620 both of which are developedinto a photoresist layer. By using a photomask configured with a dummypattern according to the invention, the pattern line width 622 of thepattern image 620 and the pattern line width 612, 622 of the first andsecond pattern image 610, 620 are similar to the original patterndimensions set in the photomask. Feature biases between patterns ofdifferent densities, i.e. of different light transmission rates, can bethereby advantageously overcome.

As described above, the dummy pattern set in the photomask is configuredto allow reduced light transmission at an adjacent area surrounding thesecond pattern so that the dummy pattern is not transferred duringexposure. This reduced light transmission reduces light scattering atthe area around the second pattern, which promotes optical accumulationat the areas of the first and second pattern of the photomask. Thedeveloped pattern images on the photoresist layer thus can have criticaldimensions consistent with the preset dimensions of the photomaskpatterns, and no compensation process thus is needed. As a result, themanufacture time and manufacture cost can be reduced.

Realizations in accordance with the present invention have beendescribed in the context of particular embodiments. These embodimentsare meant to be illustrative and not limiting. Many variations,modifications, additions, and improvements are possible. Accordingly,plural instances may be provided for components described herein as asingle instance. Additionally, structures and functionality presented asdiscrete components in the exemplary configurations may be implementedas a combined structure or component. These and other variations,modifications, additions, and improvements may fall within the scope ofthe invention as defined in the claims that follow.

1. A photolithography process comprising: providing a substrate; forminga photoresist layer over the substrate; aligning a photomask over thesubstrate, wherein the photomask at least includes a first pattern, asecond pattern and a dummy pattern surrounding the second pattern andoccupying a larger area than the second pattern, the second patternhaving a second light transmission rate different from a first lighttransmission rate of the first pattern; exposing the substrate to alight radiation through the photomask, wherein the dummy pattern isconfigured to allow light transmission in a substantially small amountso that the dummy pattern is not imaged on the photoresist layer; anddeveloping the photoresist layer.
 2. The process of claim 1, wherein thedummy pattern is dimensionally configured to include a pattern densitysubstantially smaller than a pattern density of the first and secondpatterns so that the first and second patterns are imaged while thedummy pattern is not imaged while exposing the substrate to a lightradiation through the photomask.
 3. The process of claim 2, wherein thedummy pattern has a line width smaller than a line width of the firstand second pattern.
 4. The process of claim 1, wherein the dummy patternincludes replicated features.
 5. The process of claim 3, wherein thereplicated features include hole or linear gap patterns.
 6. The processof claim 1, wherein the photoresist layer is a positive photoresist. 7.The process of claim 1, wherein the photoresist layer is negativephotoresist.
 8. The process of claim 1, wherein the photomask is made ofquartz.
 9. A photomask suitable for use in a photolithography process,comprising: a base substrate including: a first pattern of a firstpattern density; a second pattern of a second pattern density; and adummy pattern of a third pattern density surrounding the second patternand occupying a larger area than the second pattern; wherein the thirdpattern density is dimensionally configured to allow light transmissionin a substantially small amount so that the dummy pattern is not imagedin an exposure process of the photolithography process.
 10. Thephotomask of claim 9, wherein the third pattern density has a lighttransmission rate substantially smaller than that of the first andsecond pattern densities so that the first and second patterns areimaged while the dummy pattern is not imaged during exposure of thephotolithography process.
 11. The photomask of claim 10, wherein thedummy pattern includes a line width substantially smaller than a linewidth of the first and second pattern.
 12. The photomask of claim 9,wherein the dummy pattern is placed at a distance from the secondpattern.
 13. The photomask of claim 9, wherein the base substrate of thephotomask is made of a transparent material, while the first pattern,the second pattern and the dummy pattern are opaque patterns.
 14. Thephotomask of claim 9, wherein the base substrate of the photomask ismade of an opaque material, and the first pattern, the second patternand the dummy pattern are transparent patterns.
 15. The photomask ofclaim 13, wherein the base substrate is made of a transparent glass.